In generator design today, it is necessary to inspect generators intended for local and commercial energy generation for faults in the context of tests for early detection of damage. Faults may be damage in the rotor winding or damage in the stator. In the stator, damage may occur, inter alia, in the laminated cores or in the stator winding. The laminated cores are installed transversely with respect to an axis of rotation. In order to accommodate the stator winding, which generally comprises copper lines having a rectangular cross section, the laminated cores are provided with slots. The stator winding is introduced into the slots in an insulated manner. The stator winding is insulated with respect to the laminated cores.
The rotor of a generator has a rotor winding through which electrical current flows, the current level being up to 10,000 A in the case of modern generators. The electrical current is generally transmitted from an external power supply by means of sliprings and carbon brushes. In the case of limit-rating generators, the field current is produced by excitation machines having rotating rectifier wheels.
The electrical current induces a rotating magnetic field which in turn induces voltages in the stator winding located in the stator housing. An electrical current with considerably high current levels flows in the stator winding. The high current levels lead to the stator winding located in the slots being heated. The insulation between the stator winding and the laminated core may be damaged. If such damage occurs, the laminated core is heated in the local vicinity of the damaged insulation. Further damage could not be ruled out.
The insulation between the stator winding and the laminated cores is generally inspected for damage. A recent test method is to remove the rotor and to simulate an operating state which leads to heating of the laminated core. If the operating temperature is reached, an infrared recording of the stator winding and of the laminated cores is created by means of an infrared camera in a direction parallel to the axis of rotation. Faults or damage can be identified as so-called hot-spots and localized. The damage is eliminated prior to use.
Simulation of the operating state is nowadays performed by an arrangement which has a high-voltage generator and two or more phases of a field winding. The phases of the field winding are laid in parallel with the axis of rotation. The high-voltage generator is electrically connected to the field winding and produces an output voltage of up to 6 kV at a current level of approximately 700 to 800 A. The AC voltage induces a magnetic flux or an alternating magnetic field in the laminated core which leads to desired heating owing to hysteresis losses, and this heating is comparable to the heating in the operating state.
Since the generators are generally tested in situ, it is virtually impossible to have a high-voltage generator available. To date only stationary high-voltage generators have been known which cannot be used in a power station.
Until now, the laminated cores have been tested in the factory. A further possibility for testing the stator winding and the laminated cores is the so-called EL-CID test which provides, however, limited information in the case of insulated laminated stator cores.